Ultrasound has been shown to effect regional blood flow in both skeletal and cardiac muscle. In an effort to define the mechanism of action, a system has been assembled which provides capabilities for studying the microcirculation while exposed to an ultrasonic field. The major objective of the project is to determine the minimal ultrasound intensity (spatial average) which elicits a response in both the normal and ischemic microvascular bed of muscle. The physiological parameters studied include: vessel dimensions, RBC velocity, systemic pressure, and local tissue temperatures. The cremaster muscle in rats will be studied with normal perfusion and with reduced flow to create an ischemic muscle.